1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, a lithium secondary battery having an improved electrical connection structure between a safety device of the battery and a can.
2. Description of the Related Art
In general, lithium secondary batteries are rechargeable and can be made into a smaller size with high capacity. The lithium secondary batteries are widely used in advanced electronic devices, such as mobile phones, camcorders, notebook type computers and the like because of their various advantages, including high operating voltage and high-energy density.
A lithium secondary battery generally has a generation element comprising a positive electrode plate, a negative electrode plate and a separator, (i.e., an electrode unit), and an electrolytic solution. The generation unit is disposed in a can. The can is made of aluminum or aluminum alloy. A cap assembly seals an upper opening of the can. The can is made of aluminum or an aluminum alloy because aluminum is lightweight, which is advantageous in attaining lightweight batteries. Aluminum is also highly resistant to corrosion even when it is used for a long time at high voltages when compared to iron or other conductive metals. The lithium secondary battery generally has an electrode terminal formed at its upper portion and insulated from the can. The electrode terminal serves as a first electrode of the battery. In this case, the can of the battery (e.g., the bottom surface of the battery) serves as a second electrode of the battery.
In the event of external short-circuit or internal short-circuit due to mechanical impacts or overcharging, the lithium secondary battery is prone to rupture due to a sharp increase in the voltage of the battery. To avoid such a danger, the lithium secondary battery is generally electrically connected to a safety device, such as a positive temperature coefficient (PTC) element, a thermal fuse or a protecting circuit, and then encased in a battery pack. Such a safety device prevents rupture of a battery by interrupting the current flow when the voltage of the battery sharply increases.
The safety device of a battery is connected to the positive and negative electrodes of the battery through a lead. The lead is generally made of nickel, a nickel alloy or nickel-plated stainless steel to provide a predetermined level of hardness and conductivity. However, a lead made of nickel or a nickel alloy may cause several problems when it is welded to a can made of aluminum or an aluminum alloy. In other words, the infusibility of nickel makes it difficult to perform ultrasonic welding, and the high electrical, thermal conductivity of aluminum makes it difficult to perform resistance welding due to difficulty of gaining intensive heat at the contact interface. Thus, laser welding may be employed. During laser welding, however, laser beams may be transferred to the safety device, resulting in poor reliability.
To overcome the above problems, as shown in FIG. 1, an example of which is disclosed in U.S. Pat. No. 5,976,729, a cell having a safety device such as a protector 4 is connected thereto such that a bottom plate 2 made of nickel or a nickel alloy is laser-welded to a bottom surface 1a of a can 1 made of aluminum or an aluminum alloy. A lead 3 is welded to the bottom plate 2 with a welding device 5 by resistance welding. However, according to the disclosed cell, since the bottom plate 2 used in connecting the lead 3 is made of nickel or a nickel alloy having relatively higher electrical resistance than aluminum, a voltage drop of the overall battery may increase due to an increase in electrical resistance.
Also, since there is no specific basis for a welding position of the lead 3 when the lead 3 is welded to the bottom plate 2, it is necessary to constantly maintain the welding position of the lead 3, which involves an additional step and deteriorates workability.